and where will they pay with these currencies? I doubt that you can get your daily supermarket needs with tax-evasion currencies.

They can convert them into other currencies before going to the supermarket. However, of course I think we need pay taxes. Without taxes and central banks, we are going to get a complete anarchy. But that is kinda where the world is heading... People are not so smart. Maybe after the current economic system has collapsed, we can start using a less anonymous cryptocurrency.